This invention relates to electrooptic waveguide assemblies and especially to an electrooptic modulator/switch in which the effect of the modulating electric fields on the TE and TM modes propagating through electrooptic waveguides is balanced.
It is well known that the magnitude of index of refraction can be changed by applying an external electric field in some crystals. This property has been widely utilized for the switching and modulation of guided as well as unguided (bulk) optical beams. In general, selection of proper crystal orientation is a prerequisite for successful operation of electrooptic devices since the electrooptic effect is anisotropic.
Over the past decade, fiber optics and integrated optics have been vigorously developed in an attempt to realize compact, high-capacity, optical, information-processing devices for optical communication. The idea is to develop various active devices for optical-beam switching, modulation, or multiplexing, and to link the active devices by optical fibers over substantial distances, thus forming optical communication networks. Throughout such networks, the optical beam is guided by optical waveguides for the purpose of optical energy confinement and directional guidance.
Recently, it was found that in optical fibers the polarization (the direction of the electric field of the optical beam) of guided modes is unpredictably fluctuating: the guided mode does not maintain linear polarization but rather an unknown elliptical polarization. (Reference: "Performance limitations imposed on optical waveguide switches and modulators by polarization, "Applied Optics, Vol. 15, p. 2440, 1976, by R. A. Steinberg and T. G. Giallorenzi). Such elliptical polarization occurs because optical fibers have a circular cross-section which, due to slight stress or disturbance, allows the polarization to rotate.
On the other hand, in channel waveguides, there exists two orthogonal propagation modes the polarizations of which are (predictably) definite. Especially, in isotropic waveguides and cubic orientation anisotropic waveguides, the eigenmode polarization is either mainly parallel or mainly perpendicular to the waveguide surface. As a result, for beam coupling from fiber to channel waveguides, the beam from the fiber generally excites both orthogonal modes, each having different polarization. Since, as mentioned above, the electrooptic effect is anisotropic in general, one propagation mode experiences greater electrooptic effect (i.e., index change) than the other. Thus, simultaneous switching or modulation of both propagation modes is not achievable unless some specific means is devised to correct or balance the electrooptic effects for both modes.